Include addon styles for MU apps

[ppcano]

1. The PR supersedes use module unification layout for styles #8134.

This change fixes the CSS stylesheets that require importing from addons feature for MU applications importing styles from classic addons.

2. This PR also supersedes Fix MU addon styles #8146 to allow MU applications importing SASS modules like the ember-styleguide.

MU applications will now execute preprocessCss on packageStyles like Classic applications and it removes the existing preprocessCss call of the processSrc method.

I am not sure this is the correct implementation but this implementation looks more natural and it enables the specifying custom output paths works properly test at 6c65235 (cc @twokul).

A similar issue was reported at #8197 where MU applications executed preprocessTemplates twice on processSrc and processTemplates.

Closes #8134, #8133, #8146

[ppcano]

@patricklx I think the commit 183269e in this PR fixes the same issue as #8146. I will continue investigating because I think the problem is in the pre-processor logic.

[NullVoxPopuli]

anything left on this?

[ppcano]

No, as far as I know.

[ppcano]

@NullVoxPopuli who can finally review or merge this one?

This PR fixes a few reported bugs and change slightly the current MU preprocess logic.

[NullVoxPopuli]

I don't remember if I've tested it, but, @ppcano, this repo should work when pointed at your branch: https://github.com/NullVoxPopuli/test-ember-paper-with-module-unification 🤷‍♂️ can't test it out atm.
package.json is somewhat old: https://github.com/NullVoxPopuli/test-ember-paper-with-module-unification/blob/master/package.json#L25

[NullVoxPopuli]

but, I'd say either @rwjblue or @Turbo87 ?

[Turbo87]

sorry, I don't know enough about MU to be able to review this properly

[rwjblue]

Only minor changes needed here (to make future debugging of this pipeline easier), but overall looks great!

[ppcano]

@rwjblue Done

[lolmaus]

Thx, @ppcano! 🙇